U.S. Pat. No. 2,559,220 issued July 3, 1951 shows that the wet strength of cellulose paper can be improved by adding to an aqueous suspension of cellulose paper-making fibers a suitable quantity of an aqueous colloidal solution of a cationic melamine-formaldehyde resin as a binder, the preparation and constitution of which resin is shown in the U.S. Pat. No. 2,345,543 of Mar. 28, 1944.
Attempts were made to produce a glass fiber paper by adding such aqueous colloidal solution of that cationic melamine-formaldehyde resin to an aqueous dispersion of paper-making glass fibers to try to provide a glass paper by wet lay procedure.
However, that was found to be ineffective because that aqueous binder solution was unable to wet the glass fibers to adhere to them, so that most to all of melamine-formaldehyde resin binder solution drained out from the glass fibers through and down out of the Fourdrinier screen. Attempts were made to overcome that difficulty by admixing cellulose fibers with the glass fibers and using the same melamine-formaldehyde resin, but the difficulty was not overcome. Apparently, the resin binder caused the cellulose fibers to agglomerate with the result that it was impossible to obtain a useful uniform web.
Urea-formaldehyde and phenol-formaldehyde resin systems are used as binders in the dry lay method of making glass paper, by spraying a solution of the prepolymer (i.e. uncured) stage of the resin onto a dry layer of glass fibers as formed on the moving Fourdrinier screen. A phenol-formaldehyde prepolymer system has been sprayed onto a glass fiber layer formed from an aqueous slurry on a Fourdrinier screen.
In both of the foregoing cases the concentration of the prepolymer resin used exceeded 12% of the weight of the glass fiber layer, and the curing or cross-linking temperature zone was held at 248.degree. C. in a hot air oven about 200 feet long or in a high temperature curing heating zone with the curing temperature provided by direct heat from an open flame into the inner surface of a steel roller contacted by passage of the glass layer over it in an oven held at about 260.degree. C. In those cases the evolution of obnoxious and dangerous vapors of formaldehyde and phenol from the curing zone was very apparent.
Emulsions of polystyrene, polyacrylates, and of polyvinyl chloride, each separately, are used in making glass webs, but to be effective each has to be applied at a concentration exceeding 20%. Then after spraying the glass paper web, hot calendaring is required to attain significant wet strength. As a result the glass matrix is extensively blinded with consequent little or no utility as a filter.
The foregoing difficulties, shortcomings and disadvantages of these various foregoing efforts are overcome by the provision of the glass fiber filter mat of this invention.